With the explosive growth of wireless technology, there are many instances where the signal from a nearby wireless transmitter tends to overwhelm an adjacent receiver. Thus, it is ideal to remove the interfering transmitted frequencies by placing a bandstop, or notch, filter at the output of the transmitter to eliminate harmonic and spurious signals due to nonlinearity of active components such as a power amplifier. Bandstop filters can also be placed at the receiver front-ends to remove interferences due to adjacent receive bands and jammers. It is desirable for a bandstop filter to absorb the rejected signals rather than reflect them back to the previous stage, since at low RF power levels, the reflected signals could interact with the transmitted signals to create interferences known as intermodulation distortion products. At high RF power levels, the reflected energy could even physically damage the transmitter. It is also desirable to electronically tune the rejection frequency more than an octave bandwidth, since the strongest harmonics are usually two or three times higher than the transmitted signal frequency. In addition it is a very desirable attribute for the bandstop filter to have the flexibility to tune the rejection bandwidth as well.
There are many YIG based tunable bandstop filters that are commercially available, which can be tuned over a wideband with minimal insertion loss. However, YIG based tunable bandstop filters are bulky and their tuning speed is very slow (compared to electronic tuning). These drawbacks limit the usage of YIG based bandstop filters in current wireless applications where high integration and high speed are demanding. There have been many published methods in achieving planar bandstop filters but none have reported the ability to deliver an absorptive bandstop filter that can be electronically tuned over an octave bandwidth or greater, and none have the capability to tune both the rejection frequency and the rejection bandwidth.
For example, U.S. Pat. No. 3,895,304, entitled “Tunable Microwave Notch Filter”, Klein, Jul. 15, 1975, discloses quadrature hybrid devices to steer transmitted and reflected energy to provide an absorptive bandstop filter. This approach, however, uses a phase shifter, and therefore is inherently narrow band. Also, it does not provide the bandwidth tuning ability.
U.S. Patent Publication No. US20040183624A1, Electrically Tunable Notch Filters, Liang et al, Sep. 23, 2004, discloses a bandstop filter including a main transmission line and at least one electrically tunable resonator coupled to the transmission line. However, this device requires a large number of resonators to provide a reasonable level of rejection or wider stop band resulting in higher insertion loss over the pass-band. Also a length of ¼ wavelength is required between each two resonators leading to a relatively narrow frequency tuning solution that is physically large.
U.S. Patent Publication No. US20060273869A1, Narrow-band Absorptive Bandstop Filter with Multiple Signal Paths, Jachowski, Dec. 7, 2006, discloses bandstop filtering using directional couplers to steer signals as well as band pass filters to add and subtract signals to create the notch characteristic. While this approach provides a good absorptive characteristic, it uses a microstrip delay line phase shifter which is narrow band in nature. Also, the rejection bandwidth tuning option is not available.
U.S. Patent Publication No. US20090289744A1, Electronically Tunable, Absorptive, Low-loss Notch Filter, Miyashiro, Nov. 26, 2009, discloses a tunable absorptive bandstop filter using a four port quadrature hybrid coupler connected to a matched pair of band pass resonators and resistive terminations. While it shows low loss and high power handling capacity, the level of isolation between the first and fourth terminals became a limiting factor to the rejection level of the bandstop filter. Also relying on the quadrature hybrid coupler made it relatively large size and limited bandwidth.